Silicon wafers are typically obtained by a multi-step process, including: slicing a single crystal silicon ingot in a direction normal to the axis of the ingot to produce thin wafers; chamfering or profiling the edges of the wafers; grinding or lapping the wafers to remove surface damage caused by the slicing process; chemically etching the wafers to remove mechanical damage produced by the prior shaping steps; and finally, chemically/mechanically polishing the edge and at least one surface of each wafer with, for example, a colloidal silica slurry and a chemical etchant to ensure that the wafers have highly flat, reflective, and damage-free surfaces. The wafers are then typically cleaned and quality inspected prior to being packaged. Additional cleaning steps can be introduced between these steps as required.
Prior to chemical etching, silicon wafers typically exhibit surface and/or subsurface defects such as embedded particles and physical damage (e.g., micro-cracks, fractures, or stress-induced crystalline imperfections). The physical damage may be induced by stress applied to the wafer by processes such as lapping, grinding, and edge profiling. These defects generally occur in the region extending from the surface of the wafer to at least about 2.5 μm or greater below the surface of the wafer. To remove these defects, therefore, at least about 2.5 μm of silicon is typically removed from the surface of the wafer using an acidic and/or caustic chemical etchant, thus removing the embedded particles, contaminants, and physical damage contained in the removed layer of silicon.
Caustic etching offers certain advantages over acidic etching. Commercially available sources of sodium hydroxide, potassium hydroxide, and similar hydroxide etchants, however, typically contain a significant concentration of nickel, copper, and other metals capable of diffusing into the bulk of the silicon wafer during the etching process. To reduce the potential for contamination, therefore, Nakano et al., U.S. Pat. No. 6,110,839, pretreat the caustic etching solution with a chelating resin to reduce the metal ion concentration in the etchant before the etchant contacts the wafers in an etchant tank or equivalent apparatus; disadvantageously, however, this approach introduces an additional processing step and fails to address the problem of metallic impurities introduced into the caustic etching solution after the pretreatment (i.e., during the etching process itself).